As with MPEG-2 does, H.264 scheme, which is one of video coding schemes, defines a conformance assurance method using a virtual buffer as a standard (refer to Non-Patent Reference 1).
In the conformance assurance method, an encoder side controls a real coding amount in order not to cause buffer underflow in a decoder side. Thereby, it is assured that discontinuity does not occur in reproducing by the decoder. For this control, the encoder prepares a virtual buffer that corresponds to a decoder buffer.
The above aspect is explained with reference to FIGS. 1 and 2. In FIG. 1, a video coding device 1000 is one example of conventional video coding devices. The conventional video coding device 1000 includes an image sorting unit 1001, an orthogonal transformation unit 1002, a quantization unit 1003, a lossless coding unit 1004, a rate control unit 1006, an inverse quantization unit 1007, an inverse orthogonal transformation unit 1008, a frame memory 1009, and a motion estimation/compensation unit 1010.
These units are elements included in a video coding device according to MPEG-2, H.264, or the like, and disclosed in detail in Non-Patent Reference 2, for example. Therefore, these units are not explained here.
The processing performed by a buffer simulation unit 1005 included in the video coding device 1000 is explained with reference to FIG. 2. Symbols in FIG. 2 have the following meanings.
R denotes an input bitrate provided to a decoder buffer.
B denotes a size of the decoder buffer.
F denotes a buffer occupation amount of a buffer from which a decoder retrieves the first picture.
D denotes a delay time occurred when the decoder retrieves the first picture from the buffer.
t_i denotes a display timing of a picture {i}
b_i denotes a coding amount of the picture {i} (a result of calculating a coding amount outputted from the lossless coding unit 1004 on a picture-by-picture basis).
Here, a picture refers to a frame or a field.
Then, if F_i is assumed as a buffer occupation amount immediately before a picture coding amount b_i is retrieved at a timing t_i, the following equations are established.F—0=F   (Equation 1)F—{i+1}=min(B, F—i−b—i+R×(t—{i+1}−t—i))   (Equation 2)
For a variable bitrate in MPEG-2, H.264, or the like, an encoder needs to perform coding in order to satisfy the following equation.F—i−b—i≧0   (Equation 3)
This equation means that “the buffer holds bits that are equal to or more than (b_i) to be retrieved”. If the conditions are satisfied, a decoder does not get the situation where necessary bits are not available in decoding. As a result, discontinuity does not occur in reproduced images.    [Non-Patent Reference 1] ITU-T H.264 (03/2005): Infrastructure of audiovisual services-Coding of moving video, Advanced video coding for generic audiovisual services    [Non-Patent Reference 2] “H.264/AVC Kyokasho”, Okubo et., Impress Corporation, 2004    [Patent Reference 1] Japanese Unexamined Patent Application Publication No. 2006-33014